Basically getting banged around. It's like a series of rapid blows. In the case of aircraft it's where everything just rapidly swings from side to side.
Eh, no, the pre-stall buffeting in small aviation is more like driving on small rubble, or like touching some of that vibrating paint marking the side of a highway, maybe a bit stronger than that. Some aircraft do not buffet at all when approaching critical angle of attack, which is actually worse for safety - you're deprived of a useful signal. And regardless of whether they buffet or not the standard stall warning in big aviation consists of shaking/vibrating the stick/yoke (called a "stick-shaker"). Every professional pilot is intimately familiar with that kind of feedback from their training on smaller aircraft and will instinctively push on their controls mostly without thinking. Unless they actually want to stall the plane, for example for aerobatics.